This invention concerns calcium/aluminum alloys and their method of preparation.
Calcium and aluminum are used by the metallurgical industry for a variety of purposes. In ferrous metallurgy, for example, they are used extensively as addition agents, generally alone, but sometimes together or in combination with other elements such as lithium, for such functions as to deoxidize, desulfurize and degas steel and cast iron; to control the type and distribution of nonmetallic inclusions in steel; and to promote a uniform microstructure in gray iron.
Of particular importance today is their incorporation into lead, generally together with tin, to form the grids for maintenance-free auto batteries. The addition of calcium to the lead not only aids in fabrication of the grid but also markedly reduces gassing during battery operation, thus permitting the battery to be sealed. The presence of aluminum and tin further improves the mechanical and electrical properties of the grid.
The addition of calcium to other metals is especially problematical because calcium is highly reactive and the addition process is usually violent. For example, an explosive reaction accompanied by smoke and flare is encountered when pure calcium metal is added to molten steel. In addition, calcium is highly reactive with atmospheric moisture, turning to a hydrated form of lime within a short period of time. Because the metal is very ductile, it is not easily pulverized for use in such as gaseous injection processes.
Certain means of alloying calcium with aluminum are known. Thus, for example, the reduction of calcium oxide by molten aluminum is employed in U.S. Pat. Nos. 2,257,988 and 2,955,936 to prepare such an alloy containing a minor proportion of calcium, while the electrolysis of a calcium aluminate dissolved in a molten bath of alkali, alkaline earth or magnesium metal halides, as described in U.S. Pat. No. 2,829,092, purportedly results in the alloy containing any proportion of calcium to aluminum. Such preparations are commercially impractical and expensive, however. Preparation of a powdered calcium/aluminum alloy containing from 15 to 35 weight percent calcium by direct combination of the two metals has been proposed in Japanese Kokai No. 149310/79; as disclosed, calcium is added to molten aluminum under a nitrogen or argon atmosphere and the resulting melt is stirred, cast and crushed. In this range of compositions and method of addition, the problems of calcium reactivity and solids precipitation during alloy formation are not encountered. U.S. Pat. No. 2,829,092 also suggests combining the elemental forms of the metals to produce the alloy, but the process details are not revealed. Therefore, despite such disclosures, the need still exists for a simple, inexpensive process for preparing a homogeneous calcium/aluminum alloy containing a major proportion of calcium which can be readily handled and used.
Prior preparations of tertiary and higher alloys containing calcium and aluminum generally have utilized separate addition of the calcium and aluminum rather than calcium/aluminum alloy. Thus, U.S. Pat. Nos. 3,920,473 and 3,939,009 disclose the preparation of lead/calcium/aluminum alloy for battery plate grids in which a molten alloy of lead and aluminum is mixed with a solid alloy of lead and calcium, the lead/calcium alloy being obtained by the addition of calcium hydride to molten lead. In British Pat. No. 1458016, the alloy is prepared by adding solid calcium below the surface of a lead/aluminum melt protected by a layer of aluminum and aluminum oxide to prevent oxidation of the calcium, while U.S. Pat. No. 4,233,070 discloses the preparation of the alloy by the simultaneous addition of calcium and aluminum to molten lead. In U.S. Pat. No. 4,125,690, lead/tin/calcium/aluminum alloy is prepared by adding a solid lead/calcium/aluminum alloy to molten lead and then adding solid tin.
U.S. Pat. No. 4,286,984 discloses an alloy of calcium and aluminum with iron and/or manganese obtained by either adding the iron and/or manganese to molten calcium/aluminum eutectic alloy or by adding calcium to molten iron/aluminum, manganese/aluminum or iron/manganese/aluminum alloy, but the preparation of the calcium/aluminum alloy is not revealed.
It is therefore a primary objective of the present invention to provide a calcium/aluminum alloy containing a major proportion of calcium which is readily handled and used, to provide a simple and inexpensive process for preparing such an alloy by direct combination of the calcium and aluminum in elemental form, and to employ the alloy thus produced in the preparation of ternary alloys with such as lithium, lead and tin.